I'm fairly new at it, but I think your question is much like asking "Is buying a car worth it?" To some, no...it would be unnecessary. To most, yes...though that hardly begins to answer the question "What car should I buy", or translating the analogy, how and how much should I spend on AdWords. When someone comes up with the universal answer to that question, the world will beat a path to their doorstep.
Adwords is the first choice many decide to join because of the large market share. With that comes competition. Keywords are often more expensive than other engines due to the competition. Some have seen better results with MSN adCenter or Yahoo (like me!). Regardless, if you dont advertise on adwords you are missing a large part of the market (that you might be able to pick up organically).
I've been using AdWords to try to get affiliate commissions and have not had it work so well. I point clickers either to my website or directly to an affiliate page. I get clicks, but nowhere near enough sales to cover the AdWords cost. With money to play around with, you can experiment and make it work, and it would be worth it. But I don't know how many people hit a home run on their first trip to the plate.